Consolidate self user info API requests

[leafty]

Information about the user is queried in different ways:

• /ui-server/api/data/user -> provides user information from data-services
• /ui-server/api/kc/realms/Renku/protocol/openid-connect/userinfo -> provides user information from Keycloak
• /ui-server/api/user -> provides user information from the internal GitLab instance

See SwissDataScienceCenter/renku-data-services#287: we should consolidate the first and second API requests to only query data-services. This will be possible once data-services provides the is_admin field.

Also note that /ui-server/api/user should be moved down the App tree to only happen in Renku 1.0 routes in the future.